Human hair, like animal wool, horn, nails, skin, and feathers, etc., comprises proteinaceous helices known as keratins. Such structural proteins degrade with prolonged exposure to sunlight, harsh chemicals such as dyes and bleach, and air-borne pollutants. Hair follicles also stop producing the requisite melanin as a person ages; thus, the hair turns gray. To preserve a youthful appearance or for fashion purposes, the cosmetic industry has developed conditioners and coloring agents for hair. In addition, fragrances and UV blockers have been incorporated into shampoos and conditioners to further impart desirable attributes. However, the technical approaches traditionally adopted to achieve these objectives have been developed in an ad hoc fashion.
Current hair dyes and dying systems involve harsh chemicals, such as oxidizing agents, to convert pigment precursors into colored species after such precursors are first applied to the hair. This basic approach requires the precursors to penetrate deeply into the hair shaft, whereupon the oxidative conversion takes place in a subsequent operation. Similarly, when lighter colors or shades are desired, the bleaching agents must diffuse deeply into the hair to destroy the intrinsic melanin deposits. Repeated dying or bleaching using harsh chemicals tends to damage the hair significantly. Scalp exposure to the chemicals also may induce allergic reactions in sensitive individuals.